Seasonal Affective Disorder (SAD, winter depression) is a relatively common form of depression. The standard therapy for SAD, exposure to bright light for one half hour or more each morning during symptomatic episodes, is highly effective for most patients but is hardly convenient, therapeutically inadequate for some, and possibly unsafe for patients with certain retinal diseases. Light applied to the back of the human knee, rather than to the eye, has recently been shown to shift circadian rhythms: since light treatment for SAD may act through such shifts, such extraocular light may be an effective therapy for SAD. If a lightweight, compact, wearable source for extraocular light were available and shown to be effective in SAD, some of the significant limitations of bright light therapy might be overcome. Patients might receive therapy while engaged in their usual activities. Such therapy might be applied during sleep, when sensitivity to circadian phase shifts is maximal, and be more effective than conventional bright light therapy. Lastly, extraocular light would not expose the eye to bright light which might be contraindicated in certain retinal disorders. This project will produce a wearable unit for applying light to this area using light-emitting diodes, will test the device's efficacy in treating SAD in a placebo-controlled, double blind randomized trial, and explore its efficacy in sleep as well as waking hours. PROPOSED COMMERCIAL APPLICATIONS: If the proposed device is effective in treating SAD, a simple, inexpensive, convenient unit would be commercially available for millions of individuals who suffer from this disorder. Such a unit could also be useful in treatment of other seasonal psychiatric disorders, circadian rhythm disorders such as sleep syndromes, and in some instances non-seasonal depression.